Advanced thermoelectric applications for high efficiency thermoelectric materials include solid state thermoelectric devices for converting thermal energy into electrical energy, and for cooling using electricity. Thermoelectric materials may be used in an electrical circuit between a high temperature junction and a low temperature junction. For thermoelectric power generation, a temperature difference between the junctions is utilized to generate electrical energy; while in thermoelectric cooling, electrical energy is used to transfer heat from a cold junction to a hot junction. Thermoelectric technology is of interest in many areas, including but not limited to the automotive industry, due to the potential for waste heat recovery to improve fuel economy and for environmentally friendly cooling. Significant effort has been expended to develop improved thermoelectric materials since the performance of a thermoelectric device depends, at least in part, on the thermoelectric material properties.
The energy conversion efficiency and cooling coefficient of performance (COP) of a thermoelectric (TE) device are determined by the dimensionless TE materials' figure of merit, ZT, defined as
      ZT    =                                        S            2                    ⁢          T                          ρκ          total                    =                                    S            2                    ⁢          T                          ρ          ⁡                      (                                          κ                L                            +                              κ                e                                      )                                ,where S, T, ρ, κtotal, κL, and κe are the Seebeck coefficient, absolute temperature, electrical resistivity, total thermal conductivity, lattice thermal conductivity and electronic thermal conductivity, respectively. The larger the ZT values, the higher the efficiency or the Coefficient of Performance (COP). It is desirable that good thermoelectric materials possess a large Seebeck coefficient, a low electrical resistivity, and a low total thermal conductivity. The Seebeck coefficient (S) is a measure of how readily the respective charge carriers (electrons or holes) can transfer energy as they migrate through a thermoelectric material that is subjected to a temperature gradient. The type of charge carriers, whether electron or hole, depends on the dopants (N-type or P-type) in the semiconductor materials used to form the thermoelectric materials.
In an effort to increase ZT, many material exploration and optimization investigations have been undertaken to lower the lattice thermal conductivity (κL) without deteriorating the power factor (S2/ρ). For example, in thermoelectric materials such as skutterudites, clathrates and chalcogenides, all of which have a microscopic cage-like structure, guest ions interstitially inserted into the voids of the crystal lattice of the materials exhibit large atomic displacement parameters. These guest ions, termed “rattlers”, interact with low-frequency lattice phonons. This interaction significantly reduces κL, leading to substantial ZT increases at both low and high temperatures. Other methods of enhancing ZT have included the introduction of simultaneous isoelectronic alloying and doping on different crystallographic sites (in the case of half-Heusler structures).
It has been recently demonstrated that a large enhancement of the Seebeck coefficient may be achieved in nanowires. Nanowires alone, however, are unlikely to be used for practical TE devices.
It would be desirable to provide a thermoelectric device utilizing a composite material having an overall low thermal conductivity, in addition to the TE material incorporated therein having a desirably high thermoelectric figure of merit (ZT). It would further be desirable to provide such a device which is suitable for use in a wide range of applications, from low temperature applications to high temperature applications.